feat(constants): add max width for authentication pages

- Introduce new constant kMaxAuthWidth with a value of 400
- This constant can be used to ensure consistent max width across all authentication pages in the application

Author: fulleni

lib/shared/constants/app_constants.dart

@@ -3,6 +3,9 @@ abstract final class AppConstants {
   /// The maximum width the application should occupy on large screens.
   static const double kMaxAppWidth = 1000;
 
+  /// The maximum width for the authentication pages.
+  static const double kMaxAuthWidth = 400;
+
   /// The duration for debouncing search input, to prevent excessive API calls.
   static const Duration kSearchDebounceDuration = Duration(milliseconds: 300);